(1) Field of the Invention
The present invention relates to an adhesive film for semiconductor package. More particularly, the present. invention relates to an adhesive film for semiconductor package which is superior in heat resistance and reliability.
(2) Description of the Prior Art
Various adhesives have generally been used for adhesion of the members of a semiconductor device, such as lead pin, substrate for mounting of semiconductor chip, radiator, semiconductor chip and the like. These adhesives include, for example, an adhesive comprising an epoxy group-terminated modified polyamide resin (JP-A-5-51447); an adhesive comprising an epoxy resin and a polyimide resin having polysiloxane units (JP-A-6-200216); and an adhesive comprising an imide type resin (JP-A-10-135248).
However, these adhesives have respective problems. The adhesive comprising an epoxy group-terminated modified polyamide resin has insufficient heat resistance because the modified polyamide resin used therein has a low glass transition temperature, and shows no sufficient adhesivity particularly to the glossy surface of copper foil. The adhesive comprising an epoxy resin and a polyimide resin having polysiloxane units is insufficient in the adhesivity of polyimide resin and the resistance to; soldering heat. The adhesive comprising an imide type resin has a problem in that it tends to cause cracking in the heat shock test conducted for the semiconductor package obtained.
The present invention aims at alleviating the problems of the prior art, i.e. inferior heat resistance, adhesivity, resistance to soldering heat and resistance to repeated heating and providing an adhesive film for semiconductor package, superior in reliability.
According to the present invention, there is provided an adhesive film for semiconductor package, which comprises a polycarbodiimide resin, an epoxy resin and an inorganic filler, wherein the polycarbodiimide resin has a polystyrene-reduced number-average molecular weight of 3,000 to 50,000 as measured by gel permeation chromatography, the epoxy resin is contained in an amount of 20 to 150 parts by weight per 100 parts by weight of the polycarbodiimide resin, and the inorganic filler is contained in an amount of 30 to 70% by weight based on the total resin content.
The present invention is described in detail below.
The adhesive film for semiconductor package according to the present invention is composed essentially of a polycarbodiimide resin, an epoxy resin and an inorganic filler. As this polycarbodiimide resin, there can be used those produced by various methods. There can be used isocyanate-terminated polycarbodiimides produced fundamentally by the conventional method for producing a polycarbodiimide [U.S.P. 2,941,956; JP-B-47-33,279; J. Org. Chem., 28, 2069-2075 (1963); Chemical Review 1981, Vol. 81, No. 4, pages 619-621], specifically by the carbon dioxide removal and condensation reaction of an organic polyisocyanate.
In the above-mentioned method, as the organic 15 polyisocyanate which is the starting material for synthesizing the polycarbodiimide compound, there can be used, for example, aromatic polyisocyanates, aliphatic polyisocyanates, alicyclic polyisocyanates and mixtures thereof, and specifically, there can be mentioned 1,5-naphthalene diisocyanate, 4,4xe2x80x2-diphenylmethane diisocyanate, 4,4xe2x80x2-diphenyldimethylmethane diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate, hexamethylene diisocyanate, cyclohexane-1,4-diisocyanate, xylylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane-4,4xe2x80x2-diisocyanate, methylcyclohexane diisocyanate, tetramethyl-xylylene diisocyanate, 2,6-diisopropylphenyl diisocyanate, and 1, 3,5-triisopropylbenzene-2,4-diisocyanate.
Among them, those obtained from at least one aromatic polyisocyanate are preferable as the polycarbodiimide resin to be used in the present invention. Incidentally, the aromatic polyisocyanate refers to an isocyanate having, in the molecule, at least two isocyanate groups bonded directly to the aromatic ring.
As the above-mentioned polycarbodiimide, there can also be used those polycarbodiimides whose terminals are blocked with a compound (e.g. a monoisocyanate) reactive with the terminal isocyanates of polycarbodiimide and whose polymerization degrees are controlled at an appropriate level.
As the monoisocyanate for blocking the terminals of polycarbodiimide to control the polymerization degree thereof, there can be mentioned, for example, phenyl isocyanate, tolylene isocyanate, dimethylphenyl isocyanate, cyclohexyl isocyanate, butyl isocyanate, and naphthyl isocyanate.
As the other compounds reactive with the terminal isocyanates of polycarbodiimide, there can be used, for example, aliphatic compounds, aromatic compounds or alicyclic compounds having -OH group (such as methanol, ethanol, phenol, cyclohexanol, N-methylethanolamine, polyethylene glycol monomethyl ether, polypropylene glycol monomethyl ether and the like), xe2x95x90NH group (such ad diethylamine, dicyclohexylamine and the like), xe2x80x94NH2 group (such as butylamine, cyclohexylamine and the like), xe2x80x94COOH group (such as propionic acid, benzoic acid, cyclohexanecarboxylic acid and the like), xe2x80x94SH group (such as ethylmercaptan, allylmercaptan, thiophenol and the like), epoxy group, or the like.
The carbon dioxide removal and condensation reaction of the above organic polyisocyanate proceeds in the presence of a carbodiimidation catalyst. As the carbodiimidation catalyst, there can be used, for example, phosphorene oxides such as 1-phenyl-2-phosphorene-1-oxide, 3-methyl-l-phenyl-2-phosphorene-1-oxide, 1-ethyl-2-phosphorene-1-oxide, 3-methyl-2-phosphorene-1-oxide, 3-phosphorene isomers thereof, and the like. Among them, 3-methyl-1-phenyl-2-phosphorene-1-oxide is suitable from the standpoint of reactivity.
The polycarbodiimide resin used in the present invention has a polystyrene-reduced number-average molecular weight of 3,000 to 50,000, preferably 10,000 to 30,000, and more preferably 15,000 to 25,000, as measured by gel permeation chromatography (GPC) regardless of whether or not the above-mentioned terminal-blocking agent is used. When the number-average molecular weight is smaller than 3,000, no sufficient film-formability or heat resistance can be obtained. When the number-average molecular weight exceeds 50,000, a long period of time is required for the synthesis of polycarbodiimide resin and, in addition, the polycarbodiimide resin varnish obtained has an extremely short pot life (service life). Therefore, such number-average molecular weights are not practical.
As the epoxy resin used in the present invention, there can be mentioned epoxy resins having at least two epoxy groups in the molecule, for example, glycidyl ether type epoxy resins, representatives of which are bisphenol A type epoxy resins, bisphenol F type epoxy resins, phenolic novolac type epoxy resins and cresol novolac type epoxy resins; alicyclic epoxy resins; glycidyl ester type epoxy resins; heterocyclic epoxy resins; and liquid rubber-modified epoxy resins. They are used alone or in admixture of two or more. Preferable are bisphenol A type epoxy resins, bisphenol F type epoxy resins, phenolic novolac type epoxy resins and cresol novolac type epoxy resins. However, the epoxy resins used in the present invention are not limited to them and all generally known epoxy resins may be used.
In the present invention, the proportions of the polycarbodiimide resin and the epoxy resin used are 100 parts by weight for the former and 20 to 150 parts by weight, preferably 40 to 130 parts by weight, more preferably 50 to 100 parts by weight for the latter. When the proportion of the epoxy resin is smaller than 20 parts by weight, the mixture of the two resin shows no improvement in heat resistance. When the proportion of the epoxy resin is larger than 200 parts by weight, the mixture has inferior film-formability.
The adhesive film for semiconductor package comprises an inorganic filler in addition to the above two resins. As the inorganic filler, there can be mentioned, for example, silica, a quartz powder, alumina, calcium carbonate, magnesium oxide, a diamond powder, mica, a fluororesin and a zircon powder.
In the present invention, the inorganic filler is contained in an amount of 30 to 70% by weight based on the total resin content, i.e. the total amount of the polycarbodiimide resin and the epoxy resin. When the amount of the -inorganic filler is smaller than 30% by weight, the resulting adhesive film has a large coefficient of linear expansion, resulting in reduced reliability of the semiconductor package obtained. When the amount is larger than 70% by weight, the resulting adhesive film has low adhesivity. Therefore, neither of these amounts can achieve the object of the present invention.
In producing the present adhesive film for semiconductor package, comprising the polycarbodiimide resin, the epoxy resin and the inorganic filler, these components are first mixed. This mixing can be conducted, for example, by mixing the components at room temperature, or by mixing them with heating,. or by dissolving the epoxy resin in an appropriate solvent and then mixing the solution with other components. There is no particular restriction as to the method for mixing.
The above-obtained mixture of the polycarbodiimide resin, the epoxy resin and the inorganic filler is then made into a film by an ordinary method, for example, by casting the mixture on a polyethylene terephthalate (PET) film subjected to a treatment for easy release, using a coater and then heating the resulting material to remove the solvent contained therein. There is no particular restriction as to the method for film making.
The term xe2x80x9cfilmxe2x80x9d used in the present specification means a film having a thickness of approximately 10 to 500 xcexcm.
In using the thus-obtained adhesive film for semiconductor package, of the present invention for adhesion of the members of a semiconductor device, such as lead pin, substrate for mounting of semiconductor chip, radiator, semiconductor chip and the like, the adhesive film for semiconductor package is cut into a predetermined size; the cut film is interposed, for example, between semiconductor elements and a lead frame or between a ceramic substrate and a lead frame; and the resulting material is subjected to a treatment of 150 to 300xc2x0 C., 0.1 to 5 kg/cm2 and 5 seconds to 10 minutes.
The adhesive film for semiconductor package according to the present invention is flexible at room temperature, becomes more flexible and moreover adhesive when heated lightly, and is cured and exhibits adhesivity when heated strongly. The use of an. inorganic filler can keep low the thermal expansion coefficient of the film.